Embodiments of the present invention relate to a multi-section centrifugal compressor. Such compressors are used to process a working fluid in the gas or vapor state. For example, such compressors can be used to compress carbon dioxide.
A multi-section compressor may have a first and a second section. The sections operate serially, with the second section processing the output of the first section.
Both sections rotate on a common axis and each one has a plurality of impellers, each having a plurality of blades. The impeller of each section are arranged serially. Therefore, the working fluid is compressed by each impeller in a sequence, from a starting pressure to a final pressure.
Each section also has a central and a peripheral zone. Indeed, each section has an inlet duct and a discharge duct that are positioned in the peripheral zone. The discharge duct of the first section is generally placed in fluid communication with the inlet duct of the second section. In other words, the second section compresses the working fluid after it has been processed by the first section.
Additionally, the compressor has a first and second discharge scroll, which gather the working fluid from the last impeller of the first and second section respectively in order to convey it to a discharge nozzle. Due to rotordynamic constraints, in order to maintain the rotor bearing span as short as possible, one or both discharge scrolls are placed circumferentially outside the return channels u-bend of the above first and second section diaphragms bundles.
Typically, the first and second sections are configured either in a “back-to-back” or in an “in-line” configuration. In the “back-to-back” arrangement the discharge scrolls of the first and second sections are located side by side in the middle of the compressor body. In the “in-line” arrangement the discharge scroll of the first section, still located in the middle of the compressor, is adjacent to the inlet of the second section. Also, the second phase discharge is placed on the opposite side of the first section inlet. However, whenever a rotor bearing span reduction is needed to achieve an acceptable compressor rotordynamic behavior, the first section discharge scroll is located outside the return channel u-bends in both the “back-to-back” and in the “in-line” arrangement. Since the discharge scroll size is imposed by aerodynamic performance requirements, the consequence is an increased diameter of the outer casing of the compressor, which in turn has a negative impact on the whole compressor weight, cost and manageability.